Copter: convert to new get_rpm() API

ArduCopter/heli.cpp

@@ -167,7 +167,9 @@ void Copter::heli_update_rotor_speed_targets()
             // set rpm from rotor speed sensor
             if (motors->get_interlock()) {
 #if RPM_ENABLED == ENABLED
-                motors->set_rpm(rpm_sensor.get_rpm(0));
+                float rpm = -1;
+                rpm_sensor.get_rpm(0, rpm);
+                motors->set_rpm(rpm);
 #endif
                 motors->set_desired_rotor_speed(motors->get_rsc_setpoint());
             }else{

ArduCopter/system.cpp

@@ -265,9 +265,10 @@ void Copter::update_dynamic_notch()
 
 #if RPM_ENABLED == ENABLED
         case HarmonicNotchDynamicMode::UpdateRPM: // rpm sensor based tracking
-            if (rpm_sensor.healthy(0)) {
+            float rpm;
+            if (rpm_sensor.get_rpm(0, rpm)) {
                 // set the harmonic notch filter frequency from the main rotor rpm
-                ins.update_harmonic_notch_freq_hz(MAX(ref_freq, rpm_sensor.get_rpm(0) * ref / 60.0f));
+                ins.update_harmonic_notch_freq_hz(MAX(ref_freq, rpm * ref / 60.0f));
             } else {
                 ins.update_harmonic_notch_freq_hz(ref_freq);
             }